1. Field of the Invention
The present invention relates to an automatic sewing machine that operates according to stitch data to form a stitch pattern on a workpiece and, more specifically, to an automatic sewing machine capable of executing a stitch back operation when the sewing thread is broken.
2. Description of the Related Art
An embroidery machine, which belongs to one of the categories of automatic sewing machines, forms a stitch pattern on a workpiece held on an embroidery frame by moving the embroidery frame based on stitch data representing the coordinates of stitch positions in the stitch pattern.
An embroidery machine provided with a thread breakage detecting mechanism stops its sewing operation automatically when thread breakage is detected. Then, the thread is threaded normally on the embroidery machine, the embroidery frame is reversed according to the stitch data so that the last complete stitch formed on the workpiece coincides with the needle, and the embroidery machine is restarted to resume sewing operation. The operation for reversing the embroidery frame according to the stitch data so that the last complete stitch coincides with the needle is called a stitch back operation. Stitch back operation is disclosed in, for example, U.S. Pat. No. 4,413,574.
The embroidery machine capable of stitch back operation, however, has a disadvantage that a stitch is liable to fray when the first stitch to be formed after resuming sewing operation is a long one. If the embroidery frame is reversed by a stitch back operation beyond the last complete stitch until the first stitch to be formed after resuming sewing operation is a short one, a large area of the stitch pattern is double-stitched. The double-stitched area rises relative to the other single-stitched area of the stitch pattern, thereby spoiling the aesthetic quality of the stitch pattern.